Investigation of reflectance, transmittance, and thermal response of skin and surface-textured skin-equivalent phantoms in the terahertz frequency range around 0.3 THz

In this study, we investigated the electromagnetic and thermal responses of a skin-equivalent phantom with surface roughness under terahertz (THz)-wave exposure. Reflectance and transmittance were measured and compared with those of porcine skin, surface roughness standards, and flat phantoms. Results showed that both reflectance and transmittance were reduced owing to surface roughness on dielectrics. To evaluate the thermal effects of THz absorption in a skin phantom, we fabricated phantoms incorporating temperature-sensitive fluorescent probes and irradiated them with 265 GHz waves at power densities of 100, 150, and 200 mW/cm². High-resolution temperature mapping using confocal laser scanning microscopy revealed that temperature elevation occurred up to a depth of approximately 1 mm. Moreover, phantoms with surface structures showed consistently higher temperature increases than flat phantoms. These findings imply that the dominant factors affecting THz-tissue interactions vary with exposure duration. Under short-term irradiation, effective optical properties such as reflectance and transmittance altered by roughness may play a more significant role. In contrast, during long-term exposure, reduced surface water content increases transmitted THz power density into deeper tissue regions, enhancing energy absorption below the surface. Careful consideration of these factors can improve the accuracy of phantom-based evaluations in THz exposure assessments.